Technical Field
This disclosure generally relates to a power supply system, and more particularly, to tracking coulombs drawn from a power source.
Description of Related Art
In many applications, it is beneficial to know how much charge has been consumed since a battery was installed or fully charged. This information may be used, for example, for estimating the remaining battery life. Coulomb counters, sometimes referred to as “gas gauges,” are often used in battery-powered circuits to monitor the usage of the battery, which is measured in coulombs (i.e., 1 coulomb=1 ampere*1 second). Graphically, if battery current is plotted on the y-axis vs. time on the x-axis, coulomb usage is represented by the area under the curve.
Battery capacity may be rated in units of amp·hour, where 1 amp·hour=3600 amp·sec=3600 coulombs. For example, a battery with a capacity of 1 A·hour, would nominally be expected to deliver 1 A for one hour, 2 A for 0.5 hour, or 0.1 A for 10 hours, etc., before being depleted.
Real world loads, however, may not be well-defined. For example, a battery connected to the input of a DC-DC converter may be subject to a load whose input current waveform has AC and DC components. Further, there may be a large dynamic range between the highest and lowest (but nonzero) instantaneous values.
In this regard, FIG. 1 illustrates an example of input current that is provided by a power source (e.g., battery) to a buck DC-DC converter. FIG. 1 demonstrates how uneven and erratic the input current drawn by a load can be. The current profile may include current spikes 102 due to inductor currents, sequences of which are sometimes referred to as bursts. There may be other components, such as gate charge current spikes 104. During the active period, there is a DC quiescent current 106. During the sleep period, there is a second quiescent current 108 that may be lower than the active quiescent current.
Accordingly, there is a bursting phase where power is delivered to the load, which is followed by a sleep phase, where power is not delivered to the load but lost nonetheless due to the quiescent sleep current 108. The burst rate is determined by the load and the output capacitor (COUT, illustrated in FIGS. 2A and 2B).
Circuits to count the coulombs at the input of the DC-DC converter presently exist, but they can be large and complicated. Moreover, prior art coulomb counters can be subject to large errors when counting coulombs at low power levels. The greatest source of this error can be the large (but finite) dynamic range of the instantaneous currents.
FIG. 2A illustrates a known approach where a coulomb counter uses a current sense resistor RSENSE 204 in series with the input VIN to create a small voltage drop across the resistor 204 that is proportional to the input current IBUCK. The profile of the current is illustrated by way of example in waveform 202 (which is a replica of the waveform of FIG. 1 discussed above. The voltage drop across the sense resistor 204 serves as an input to the coulomb counter 206, which may include an integrator to calculate and report the area under the current vs. time curve 202. For example, the LTC2941/2/3 and LTC4150 integrated circuits from Linear Technology Corporation use this technique.
Because the voltage drop across the sense resistor 204 represents an efficiency loss, the peak voltage drop across the sense resistor 204 is generally kept small (e.g. 50 mV). Further, the ratio of the highest current to the lowest current may be 100,000 (or even higher), which also prevents using a “large” sense resistor 204 because of the substantial voltage drop across the resistor 204 it would require. The limitation of using a small sense resistor 204 can create an accuracy challenge at small instantaneous currents across the sense resistor because the integrator may have a finite offset that may exceed the IR drop created by the smallest instantaneous input current. Put differently, when the current through the sense resistor 204 is low, this low current cannot be accurately detected due to the low resistance of the sense resistor 204.
Further, at light loads, the DC-DC converter may spend the majority of the time in a sleep state, which results in the highest error of calculating the coulombs drawn from the power source. In addition, due to the AC nature of the input current waveform, bandwidth issues can further affect accuracy. For example, the integrator has a finite bandwidth (i.e., the highest frequency of an input signal at which it can perform the integration with acceptable accuracy). If the input current contains frequency components higher than this bandwidth (e.g., possibly the gate charge current spikes), these additional components typically result in additional error.
The burst rate is determined by the load at VOUT and the output capacitor COUT. Both the sleep time and the burst time are proportional to COUT. For example, the sleep and burst times can be calculated by equations 1 and 2 below.
                              Sleep          ⁢                                          ⁢          Time                =                                            C              OUT                        ×                          V              RIPPLE                                            I            LOAD                                              EQ        .                                  ⁢        1                                          Burst          ⁢                                          ⁢          Time                =                                            C              OUT                        ×                          V              RIPPLE                                                          ISW              BUCK                        -                          I              LOAD                                                          EQ        .                                  ⁢        2            
Where:                COUT is the output capacitor 220 (e.g., a constant);        ISWBUCK is the current the buck converter can deliver when in burst mode (e.g., constant for a predetermined set of conditions);        VRIPPLE is the ripple in the output voltage (e.g., depends on the implementation, but typically small); and        ILOAD is the load current (e.g., application dependent).        
FIG. 2B illustrates an alternate known implementation of counting coulombs. The approach of FIG. 2B forgoes the use of the current sense resistor 204 of FIG. 1. Instead, FIG. 2B includes a coulomb counter 256 that counts only the coulombs associated with inductor current IL when the switch 258 is closed. The implementation of FIG. 2B exploits the known shape of the current profile IL 260 flowing in the inductor L based on the particular DC-DC topology in order to simplify the architecture.
Thus, instead of determining the area of the uneven and erratic profile of the current 202 of FIG. 1, the circuit of FIG. 2B essentially takes the area of triangles provided by the current profile 260. The LTC3335 integrated circuit from Linear Technology Corporation uses this technique. Advantageously, there is no efficiency loss associated with having a separate current sense resistor as in FIG. 2A. However, some of the current components (such as gate charge current spikes 104 and DC quiescent currents 106 and 108 of FIG. 1) may not be accounted for, resulting in lower (but generally predictable) accuracy at all power levels.
A commonality in both prior art implementations is that the raw input current waveform to the coulomb counter may not be “well-behaved” under the various conditions that the circuit may operate and therefore difficult to accurately calculate the coulombs drawn from a power source by a load.
Accordingly, prior art approaches may exhibit efficiency loss and may lead to large coulomb counter errors, particularly at light loads. It is with respect to these considerations and others that the present disclosure has been written.